1. Field of the Invention
The present invention relates to a variable gain amplifier for use in such as a transmission section of, for example, a CDMA wireless terminal, and a mixer and a quadrature modulator that use such a variable gain amplifier.
2. Description of the Related Art
As a code division multiple access (CDMA) scheme has come to be utilized more and more, research and development has been greatly advancing on a radio that accommodates CDMA (hereinafter referred to as “CDMA wireless terminal”). For a transmission section of the CDMA wireless terminal, it is indispensable to conduct transmission power control of 70 dB or more, so that a variable gain amplifier plays a major role in this power control. The variable gain amplifier for use in the transmission section has requirements of (1) low dissipation power, (2) low distortion, and (3) ensuring gain continuity. Specifically, “low dissipation power” means that it is possible to set to low dissipation power when a gain and an output level are low in level, “low distortion” means that a saturation input level remains unchanged even if a gain changes, and “ensuring gain continuity” means that a gain can be changed continuously.
For example, in one of conventional variable gain amplifiers, a gain is controlled by a gain control circuit having a plurality of differential amplifiers (see Japanese Patent Application Laid Open No. 2000-232328, for example). These differential amplifiers are each made of two transistors in such a configuration that a base of one of the two transistors and that of the counterpart transistor are commonly connected and bases of the other transistors are connected to different reference voltage terminals. Further, collectors of the two transistors of each of the differential amplifiers are commonly connected and a load resistor of each of the transistors is constituted of a resistor element. The commonly-connected base of one of the transistors of each of the differential amplifiers is supplied with a voltage proportional to a thermal voltage which is obtained by converting an externally applied first gain-control voltage. An output voltage of each of the differential amplifiers is supplied via a buffer to the variable gain amplifier as a second gain-control voltage. The variable gain amplifier in which a gain is controlled by such a gain control circuit has a feature of ensuring continuity of a gain with respect to an externally applied gain control voltage with its relative simple configuration and improving linearity of that gain. Hereinafter, the term “gain control voltage” means a gain control signal in the description.
Another conventional variable gain amplifier has a first differential amplification circuit constituted of a first couple of transistors and a second differential amplification circuit constituted of a second couple of transistors (see Japanese Patent Application Laid Open No. 2003-023331, for example). In this variable gain amplifier, when an externally applied gain-control voltage increases, a bias current of the first differential amplifier having a larger gain increases, so that the gain of the first differential amplifier is increased, thereby increasing an overall gain. If the externally applied gain control voltage decreases, on the other hand, a bias current of the second differential amplifier having a smaller gain increases, so that the gain of the second differential amplifier is increased and hence its influence is increased, thus resulting in an amplification circuit having a higher saturation input level and a lower gain. In this configuration, as a gain increases, a bias current of the second differential amplifier decreases, so that dissipation power can be reduced. That is, this variable gain amplifier features that dissipation power can be reduced.
In a further conventional variable gain amplifier, when converting an input voltage into a current and amplifying this converted current to output the resulting current, a quantity of the output current is controlled by changeover of switches in accordance with an externally applied gain control voltage (see Japanese Patent Application Laid Open No. H11-340760, for example). This variable gain amplifier has a feature that a dissipation current can be reduced if a gain is low.
Furthermore, in some other conventional variable gain amplifiers, a bias current generation circuit that supplies a bias current to a linear amplifier constituting a communication high-frequency power amplification circuit is comprised of a plurality of variable-current sources having different current values at starting levels, and these variable current sources are controlled by using an input control voltage. Then, the resultant controlled currents may be combined into a bias current and this combined current may change exponentially with respect to the input control voltage (see Japanese Patent Application Laid Open No. 2003-218649, for example).
A still further conventional variable gain amplifier is comprised of a mutual conductance amplifier having a degenerate impedance element and a variable-current source that is arranged to supply a current to this mutual conductance amplifier and that is configured so that the current may change simultaneously with an effective degenerate impedance of the mutual conductance amplifier, in accordance with a gain control voltage (VGC) (see Japanese Patent Application Laid Open No. 2003-229735, for example).
However, a variable gain amplifier in which a gain is controlled by a gain control circuit having a plurality of differential amplifiers has a disadvantage that a dissipation current cannot be changed for each of the gains. Another variable gain amplifier having two differential amplifiers has a disadvantage that, although a saturation input level is low at most of intermediate level gains, the saturation input level increases if an influence of the first differential amplification circuit is reduced due to an increase in bias current of the second differential amplification circuit having a smaller gain, that is, the saturation input level fluctuates. A further variable gain amplifier for controlling a quantity of a current by changeover of switches has a disadvantage that it can conduct only discrete gain control on an externally applied gain control voltage but cannot conduct continuous control thereon.
That is, the conventional variable gain amplifiers have a problem that they cannot satisfy the above-described three features of: (1) low dissipation power that low dissipation power can be set if a gain is low and hence an output level is low, (2) low distortion that a saturation input level remains unchanged even if a gain changes, and (3) gain continuity that a continuous gain can be ensured.